This invention relates to a system for the accumulation of heat from solar radiant energy. More particularly this invention relates to a system for the accumulation of heat from solar radiant energy adapted to have solar energy absorbed by absorbing means and transferred therefrom to a heat-transfer medium, which system is improved by having said heat-transfer medium divided into a few stages for thereby enabling the heating thereof to be effected in the sequence of said stages.
Of the electromagnetic wave energies, typical is the solar radiant energy. In the orbit of the earth, the mean intensity of solar radiation is about 0.1 W/cm.sup.2 (equivalent to 1,000,000 KW per kilometer), the spectral curve of solar radiant energy has the highest value in the neighborhood of 0.5.mu. of wavelength and the color temperature thereof is 5900.degree.K. Incidentally, the solar radiant energy may as well be regarded as inexhausible. If such solar radiant energy can be efficiently and directly harnessed and converted into heat, then the system will serve as a permanent energy source which entails no environmental pollution.
Heretofore various types of solar energy collectors have been proposed (examples being those disclosed in U.S. Pat. Nos. 1,880,938; 2,917,817; 3,176,678; 3,176,679 and 3,227,153).
The present inventors pursued a research and have consequently proposed a system for absorbing solar radiant energy which enables the solar radiant energy to be absorbed at a high percentage and minimizes possible loss of absorbed energy through radiation (U.S. patent application Ser. No. 402,918 abandoned). The proposed system for the absorption of solar radiant energy comprises a highly conductive basal member, a heat absorption member disposed in close contact with the external surface of said basal member, a selectively penetrating membrane permitting passage of only desired wavelengths of the electromagnetic wave energies and disposed on said heat absorption member and a heat-transfer medium circulated inside said highly conductive basal member, whereby the sun's rays are concentratedly irradiated upon the heat absorption member to have the energies absorbed in the form of heat by said heat absorption member, the absorbed heat is conveyed through said highly conductive basal member to said coolant to elevate the temperature thereof and the coolant which now has an elevated temperature is withdrawn and put to use. Said selectively penetrating membrane permits penetration of electromagnetic wave having only wavelengths (about 0.3 to 2.0.mu.m) contemplated for absorption by the present apparatus and reflects electromagnetic waves of all the other wavelengths. Accordingly, electromagnetic wave energies which have penetrated the membrane are absorbed by the heat absorption member and radiant energies radiated by the heat absorption member are again reflected back to the heat absorption member by the membrane, with the result that electromagnetic energies are absorbed with high efficiency. When it is desired to obtain heat of a high temperature by use of the solar radiant energy absorption system of the one-stage construction described above, it will suffice for this system to be provided with a reflector of a parabolic profile or other similar device designed to concentrate the solar radiant energy impinging upon a wide area into one point. It is actually difficult, however, to obtain heat energies of a high temperature as expected, because the heat-transfer medium fails to effect the desired heat-exchange to a sufficient extent even if it is delivered at once to this focus. The output by the device under discussion will sharply decline when the actual point at which the solar energy are concentrated deviates, though to the slightest extent, from the fixed focus of the parabolic reflector. Also in this respect, it proves difficult to obtain heat of a sufficiently high intensity as the output. Since the angle at which the solar radiant energy impinge upon a given area changes constantly with lapse of time, it is essential that the system be provided with a sun-chasing device adapted for the solar radiant energy to be accurately focused at one fixed point of the reflector at all times. An attempt to obtain heat energies of a high temperature with such one stage system of the aforementioned description is difficult to accomplish and proves disadvantageous from the economic point of view.
The inventors pursued a further study on devices for the absorption of solar energies. They have, consequently, arrived at a discovery that a system in which heating is effected at a few stages of successively elevated temperatures by use of heat-absorption members optimum for respective temperature ranges fixed for said stages so as to obtain heat energies of a desired high temperature finally in the last of said stages is simpler in mechanism, suffers less from possible temperature dispersion and attains the object more easily than the system wherein the heat-transfer medium is heated immediately to a high temperature in one stage.
It is, therefore, an object of this invention to provide a system for the accumulation of heat from the solar energy which permits said energy of a high temperature to be obtained economically.